Electronic limited slip differential with separation of clutch and actuator

ABSTRACT

A differential gear mechanism constructed in accordance to one example of the present disclosure can include a differential casing. A first and a second side gear can be rotatably mounted within the differential casing. A pair of pinion gears can be mounted between the first and second side gears. Both of the pinion gears can be rotatably mounted on a cross shaft that is fixed for rotation with the differential casing. A clutch pack can include a plurality of annular plates that are interleaved between a plurality of annular friction disks. The clutch pack can be arranged on a first side of the cross shaft. An actuator assembly can comprise a piston received in a piston housing. The actuator assembly can be configured to actuate the clutch pack. The actuator assembly can be arranged on a second side of the cross shaft, opposite the first side.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Patent ApplicationNo. PCT/US2014/050026 filed on Aug. 7, 2014, which claims the benefit ofU.S. Patent Application No. 61/863,142 filed on Aug. 7, 2013, U.S.Patent Application No. 61/864,119 filed on Aug. 9, 2013, U.S. PatentApplication No. 61/875,203 filed on Sep. 9, 2013 and U.S. PatentApplication No. 62/030,683 filed on Jul. 30, 2014. The disclosures ofthe above applications are incorporated herein by reference.

FIELD

The present disclosure relates generally to differential gear assembliesand more particularly to a differential gear case arrangement having aclutch and a piston actuator configured on opposite ends of thedifferential gear case.

BACKGROUND

A differential gear mechanism can be provided in an axle assembly andused to transfer torque from a driveshaft to a pair of output shafts.The driveshaft can drive the differential through the use of a bevelgear that meshes with a ring gear mounted to a housing of thedifferential. In automotive applications, a differential allows thetires mounted at either end of the axle assembly to rotate at differentspeeds. This is important when the vehicle is turning because the outertire travels over an arc of greater distance than the inner tire. Thus,the outer tire must rotate at a faster speed than the inner tire tocompensate for the greater distance of travel. The differential includesa differential case and a gear arrangement that allows torque to betransferred from the driveshaft to the output shafts while concurrentlyallowing the output shafts to rotate at different speeds as needed. Thegear arrangement can generally include a pair of side gears that aremounted for rotation with the respective output shafts. A series ofcross pins or pinion gear shafts are fixedly mounted to the differentialcase for rotation therewith. A corresponding plurality of pinion gearsare mounted for rotation with the pinion gear shafts and are in meshingrelationship with both of the side gears.

Some differential gear mechanisms include traction modifyingdifferentials. Typically, a clutch pack can be disposed between one ofthe side gears and an adjacent surface of the differential case. Theclutch pack or locking mechanism is operable to limit relative rotationbetween the gear case and the one side gear. In such differentials,engaging the clutch pack or locking mechanism (retardingdifferentiation) is achieved by one of several different approaches.Some configurations include a piston that actuates to cause the clutchpack to move between open, locked and partially locked conditions.

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

SUMMARY

A differential gear mechanism constructed in accordance to one exampleof the present disclosure can include a differential casing definingfirst and second output shaft openings that are co-axially aligned alongan axis of rotation of the differential casing. A first and a secondside gear can be rotatably mounted within the differential casing. Thefirst and second side gears can be co-axially aligned along the axis ofrotation of the differential casing. The first side gear can define afirst shaft opening configured to provide a first torque transmittingconnection with a first output shaft received within the first outputshaft opening. The second side gear can define a second output shaftopening configured to provide a second torque transmitting connectionwith a second output shaft received within the second output shaftopening. A pair of pinion gears can be mounted between the first andsecond side gears. Both of the pinion gears can be rotatably mounted ona cross shaft that is fixed for rotation with the differential casing. Aclutch pack can include a plurality of annular plates that areinterleaved between a plurality of annular friction disks. The clutchpack can be arranged on a first side of the cross shaft. An actuatorassembly can comprise a piston received in a piston housing. Theactuator assembly can be configured to actuate the clutch pack. Theactuator assembly can be arranged on a second side of the cross shaft,opposite the first side.

According to additional features, the differential gear mechanism canfurther comprise a plurality of transfer rods that couple the clutchpack and the actuator assembly. The clutch pack can further comprise afirst transfer plate. The actuator assembly can further comprise asecond transfer plate. The plurality of transfer rods can be coupledbetween the first and second transfer plates.

According to other features, the actuator assembly can further comprisea retainer and a first needle roller disposed on one side of the pistonhousing and a second needle roller disposed on an opposite side of thepiston housing. The piston can further comprise at least one o-ringmounted thereon. The at least one o-ring can be configured to slidablytraverse along the piston housing. The actuator assembly can furthercomprise a hydraulic fitting extending from an outer diameter of thepiston housing. The hydraulic fitting can be configured to communicatehydraulic fluid from a hydraulic fluid source to the actuator assembly.

In other features, the pair of pinion gears can be intermeshed with thefirst and second side gears to form a torque transfer arrangementconfigured for transferring torque between the pinion gears and thefirst and second side gears to rotate the first and second side gearsabout the axis of rotation. The torque transfer arrangement can also beconfigured to allow the first and second side gears to rotate atdifferent rotational speeds with respect to one another about the axisof rotation. The clutch pack can be concentric with the first side gear.The actuator assembly can be concentric with the second side gear.

According to other features, the clutch pack and the actuator assemblycan be configured on opposite ends of the differential casing. Thepiston can include a piston body defined around a central piston axis.The piston body can include a central piston wall, an outer diameterpiston wall and an inner diameter piston wall. The piston housing caninclude a central annular housing wall, an outer annular housing walland an inner annular housing wall that collectively define an annularpocket that receives the piston.

In other features, the differential gear assembly can include a lugformed on one of the piston and the piston housing. A groove can bedefined by a groove wall on the other of the piston and the pistonhousing. The lug can be received by the groove. Engagement of the lugand the groove wall inhibits rotation of the piston around the centralpiston axis. The lug can be formed on the piston. The groove can beformed on the piston housing. In one configuration, the lug can beformed on the inner diameter piston wall. The groove can be formed onthe inner annular housing wall. The lug can comprise a pair of lugsconfigured on the inner diameter piston wall. The groove can comprise apair of grooves formed on the inner annular housing wall. The pair oflugs can be arranged in a diametrically opposed relationship on theinner diameter piston wall. The pair of grooves can be arranged in adiametrically opposed relationship on the inner annular housing wall.The outer diameter piston wall can define a first annular groove and theinner diameter piston wall can define a second annular groove. A firsto-ring can be received by the first annular groove. A second o-ring canbe received by the second annular groove. The first o-ring can beconfigured to slidably traverse along the outer annular housing wall.The second o-ring configured to slidably traverse along the innerannular housing wall.

A differential gear assembly constructed in accordance to additionalfeatures of the present disclosure can include a differential casingthat defines first and second output shaft openings. The first andsecond output shaft openings can be co-axially aligned along an axis ofrotation of the differential casing. A first and a second side gear canbe rotatably mounted within the differential casing. A piston can have apiston body defined around a central piston axis. The piston body caninclude a central piston wall, an outer diameter piston wall and aninner diameter piston wall. The piston body can be configured to exertan axial force on a clutch pack.

A piston housing can have a central annular housing wall, an outerannular housing wall and an inner annular housing wall that collectivelydefine an annular pocket that receives the piston. A lug can be formedon one of the piston and the piston housing. A groove can be defined bya groove wall on the other of the piston and the piston housing. The lugcan be received by the groove. Engagement of the lug and the groove wallcan inhibit rotation of the piston around the central piston axis. Thedifferential gear assembly can further comprise a clutch pack. Theclutch pack can include a plurality of annular plates that areinterleaved between a plurality of annular friction disks. The clutchpack can be arranged on an opposite side of the differential casing asthe piston.

A differential gear assembly constructed in accordance to additionalfeatures can include a differential casing defining first and secondoutput shaft openings that are co-axially aligned along an axis ofrotation of the differential casing. A first and a second side gear canbe rotatably mounted within the differential casing. A pair of piniongears can be mounted between the first and second side gears. Both ofthe pinion gears cab be rotatably mounted on a cross shaft that is fixedfor rotation with the differential casing. A clutch pack can include afirst transfer plate and a plurality of annular plates interleavedbetween a plurality of annular friction disks. The clutch pack can bearranged on a first side of the cross shaft. An actuator assembly cancomprise a second transfer plate and a piston received in a pistonhousing. The actuator assembly can be configured to actuate the clutchpack. The actuator assembly can be arranged on a second side of thecross shaft, opposite the first side. A plurality of transfer rods canbe coupled between the first and second transfer plates.

According to other features, the differential gear assembly can furtherinclude a lug formed on one of the piston and the piston housing. Agroove can be defined by a groove wall on the other of the piston andthe piston housing. The lug can be received by the groove. Engagement ofthe lug and the groove wall can inhibit rotation of the piston aroundthe central piston axis.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a front perspective view of a differential gear mechanismconstructed in accordance to one example of the present disclosure;

FIG. 2 is a cross-sectional view of the differential gear mechanism ofFIG. 1 taken along lines 2-2;

FIG. 3 is a cross-sectional view of the differential gear mechanism ofFIG. 1 taken along lines 3-3;

FIG. 4 is a detail view of the actuator assembly of FIG. 1;

FIG. 5 is an exploded view of the actuator assembly of FIG. 4;

FIG. 6 is a perspective view of a piston assembly of the differentialgear mechanism of FIG. 1;

FIG. 7 is a cross-sectional view of the piston assembly of FIG. 6 takenalong lines 7-7 and shown with the piston in a relaxed state; and

FIG. 8 is a cross-sectional view of the piston assembly of FIG. 7 andshown with the piston in a fully exerted state.

DETAILED DESCRIPTION

With initial reference to FIG. 1, an electronic limited slipdifferential assembly constructed in accordance to the presentdisclosure is shown and generally identified at reference 10. Theelectronic limited slip differential assembly 10 can generally include adifferential gear assembly or mechanism 20 arranged in a differentialcase 22 and a clutch assembly 30. The limited slip differential assembly10 can be received in a housing (not shown) and operates to drive a pairof axle shafts (not shown) that are connected to drive wheels (notshown). In general, the limited slip differential assembly 10 functionsas a traditional open differential during normal operating conditionsuntil an event occurs where a bias torque is required. When a loss intraction is detected or anticipated, the clutch assembly 30 can beselectively actuated in order to generate the optimum bias ratio for thesituation.

With additional reference to FIGS. 2 and 3, the differential gearassembly 20 includes a pair of side gears 40 and 42 that are mounted forrotation with the axle shafts (and first and second drive wheels). Theside gears 40 and 42 define first and second axle shaft openings 44 and46 (FIG. 3). A cross pin or pinion gear shaft 50 can be fixedly mountedto the differential case 22 for rotation therewith. A corresponding pairof pinion gears 52 are mounted for rotation with the pinion gear shaft50 and are in meshing relationship with both of the side gears 40 and42. In an open configuration, described more fully below, thedifferential gear assembly 20 acts to allow the axle shafts to rotate atdifferent speeds.

The clutch assembly 30 couples an input of the electronic limited slipdifferential 10 with the differential gear assembly 20. In someexamples, the input can comprise a ring gear fixedly arranged around thedifferential case 22 that is driven by a pinion gear. The clutchassembly 30 can generally comprise a clutch pack 60 and a clutchactuator assembly 62.

The clutch pack 60 includes a plurality of annular plates 64 interleavedbetween a plurality of annular friction disks 68. The plurality ofannular plates 64 can be coupled for rotation with one of thedifferential case 22 and the differential gear assembly 20. Theplurality of annular friction disks 68 can be coupled for rotation withthe other one of the differential case 22 and the differential gearassembly 20. In the illustrated embodiment, the plurality of annularplates 64 are coupled for rotation to the differential case 22 (e.g.,splined to an inner diameter of the differential case 22) and theplurality of annular friction disks 68 are coupled for rotation with thedifferential gear assembly 20 (e.g., splined to an outer diameter of theside gear 40). It will be appreciated that the annular friction disks 68may be supported for rotation by either of the side gears 40 or 42, orboth. A first transfer plate 69 can be arranged as part of the clutchpack 60.

The plurality of annular plates 64 and annular friction disks 68 areinterleaved between one another and act to rotate past one another insubstantially non-contacting relationship when the clutch assembly 30 isin its open position. However, it will be appreciated by those skilledin the art that the term “non-contacting” as used herein is relative andis not meant to necessarily indicate that the annular plates 64 andannular friction disks 68 have absolutely no contact when the clutchassembly 30 is in the open condition. The annular plates 64 and annularfriction disks 68 are axially movable into frictional engagementrelative to one another, thereby reducing relative rotation between theannular plates 64 and annular friction disks 68 when the clutch assembly30 is in the closed or partially closed configurations. In this manner,when the clutch assembly 30 is in its closed position, the side gears 40and 42, as well as the axle shafts and the drive wheels rotate together.

The clutch assembly 30 can operate in an open configuration to allow theside gears 40 and 42 to rotate independently from each other, e.g., atdifferent speeds. The clutch assembly 30 can also operate in a closed orpartially closed configuration where the side gears 40 and 42 rotatetogether or partially together (that is, not independently), e.g., atsubstantially the same speed. The clutch assembly 30 can, for example,be a hydraulic clutch assembly 30 that utilizes pressurized hydraulicfluid that can act on the actuator assembly 62 to selectively actuatethe clutch pack 60 between the open, closed and partially closedconfigurations.

With particular reference now to FIGS. 2-4, the actuator assembly 62will by further described. The actuator assembly 62 is arranged on anopposite end of the differential case 22 as the clutch pack 60. In oneexample, the clutch pack 60 can be generally concentric with the firstside gear 40 and the actuator assembly can be generally concentric withthe second side gear. As a result of positioning the actuator assembly62 on an opposite end of the differential case 22 as the clutch pack 60,a number of advantages are realized. For example, the resulting packagesize of the electronic limited slip differential 10 is reduced. Thecross-shaft 50 is centered between bearing races. Stock axle shaftshaving standard lengths can be used. The clutch pack 60 can beconstructed larger to provide greater torque capacity. The electroniclimited slip differential 10 can be replaced with an “open” differentialon the same assembly line. The actuator assembly 62 can generallyinclude a piston assembly 68 having a retainer 70, a first needle roller72, a bearing race 74, a piston housing 80, a piston 82, a pair ofo-rings 84, a second needle roller 88 and a second transfer plate 90.The second transfer plate 90 can act as a bearing race for the secondneedle roller 88. The piston housing 80 can be mounted in a ring 91 thatseats on the retainer 70.

A plurality of transfer rods 92 are disposed between the second transferplate 90 of the actuator assembly 62 and the first transfer plate 69 ofthe clutch pack 60. In the example shown, eight transfer rods 92 aredisposed between the second transfer plate 90 and the first transferplate 69. The transfer rods 92 can be spaced equally around thedifferential casing 22. Other quantities of transfer rods arecontemplated. Furthermore, other structures may be used to transferforce between the first and second transfer plates 70 and 90.

With further reference now to FIGS. 5-8, additional features of thepiston assembly 68 will be described. During operation, movement of thepiston 82 leftward (as viewed in FIGS. 2-4) causes the transfer rods 92to push the first transfer plate 69 leftward causing the clutch pack 60to close. Similarly, when pressure is released from the piston 82, thetransfer rods 92 move in an axial direction rightward (as viewed inFIGS. 2-4) causing the clutch pack 60 to open. Hydraulic fluid can becommunicated from a hydraulic fluid source (not shown) to the actuatorassembly 62 through a port 98.

The piston 82 can include a piston body 100 defined around a centralpiston axis 102. The piston body 100 can include a central piston wall110, an outer diameter piston wall 112 and an inner diameter piston wall114. The outer diameter piston wall 112 can define a first annulargroove 120. The inner diameter piston wall 114 can define a secondannular groove 122. The first o-ring 84 can be received by the firstannular groove 120. A second o-ring 130 can be received by the secondannular groove 122. The first and second o-rings 84 and 130 can beconfigured to slidably traverse along the piston housing 80.

The piston 82 can define a pair of lugs 140 extending from the innerdiameter piston wall 114. The lugs 140 are arranged in a diametricallyopposed relationship on the piston body 100. As will become appreciatefrom the following discussion, while the lugs 140 are shown as a pair oflugs around the inner diameter piston wall 114, other quantities of lugs140 and other locations of the lugs 140 are contemplated.

The piston housing 80 will now be further described. The piston housing80 can generally include a central annular housing wall 150, an outerannular housing wall 152 and an inner annular housing wall 154. Thecentral annular housing wall 150, the outer annular housing wall 152 andthe inner annular housing wall 154 can collectively define an annularpocket 156 that receives the piston 82. The piston 82 is configured toride within the annular pocket 156 upon introduction of hydraulic fluidbetween the piston 82 and the piston housing 80. The o-rings 84 and 130sealingly engage the piston housing 80. During operation, the pistonhousing 80 does not rotate.

The piston housing 80 can include a pair of groove walls 160 that definea pair of corresponding grooves 162. In the example shown, the grooves162 are formed on the inner annular housing wall 154 in a diametricallyopposed relationship. The grooves 162 are provided in a reducedthickness portion formed on the inner annular housing wall 154.

As will become appreciated from the following discussion, while thegrooves 162 are shown as a pair of grooves around the inner annularhousing wall 154, other quantities of grooves and other locations ofgrooves are contemplated. Moreover, while the lugs 140 are shown as partof the piston 82 and the grooves are shown as part of the piston housing80, the features may be reversed. Alternatively, the piston 82 mayincorporate one or more lugs in addition to one or more grooves whilethe piston housing 80 may incorporate one or more lugs in addition toone or more grooves. Regardless, the relationship of the lugs 140 andthe grooves 162 inhibits rotation of the piston 82 around the centralpiston axis 102. In this regard, the lugs 140 can engage the groovewalls 160 restricting rotation of the piston 82 around the centralpiston axis 102 (FIG. 6).

As identified above, the piston assembly 62 can further include ahydraulic port 98 configured on the piston housing 80. The hydraulicport 98 can be configured to communicate hydraulic fluid from the pistonassembly 68. In the example shown, the hydraulic port 98 is formed onthe outer annular housing wall 152. The hydraulic port 98 can extendalong a longitudinal port axis 172. The longitudinal port axis 172 canbe transverse to the central piston axis 102 (FIG. 6). The hydraulicport 98 can communicate hydraulic fluid from a hydraulic fluid source(not shown) to the actuator assembly 62.

During operation, the piston 82 can move from a relaxed state (FIG. 7)to an exerted state (FIG. 8). As shown in FIGS. 7 and 8, in both states,the lugs 140 on the piston 82 remain engaged with the grooves 162 on thepiston housing 80 thereby inhibiting rotation of the piston 82 aroundthe central axis 102 (FIG. 6). In some implementations, axial forces canbecome large enough to rotate the piston 82 within the piston housingpocket 156. Such a condition is undesirable as it may cause galling,premature wear and/or tearing of the sealing features of the pistonassembly 62. The lug and groove features provided by the piston assembly62 of the present teachings eliminates such a condition. In otherbenefits, the piston assembly 68 provides anti-rotation features withoutincreased components. In this regard, additional components such as, butnot limited to, dowel pins, clips and fasteners are not needed.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

What is claimed is:
 1. A differential gear assembly comprising: adifferential casing defining first and second output shaft openings thatare co-axially aligned along an axis of rotation of the differentialcasing; a first and a second side gear rotatably mounted within thedifferential casing, the first and second side gears being co-axiallyaligned along the axis of rotation of the differential casing, the firstside gear defining a first shaft opening configured to provide a firsttorque transmitting connection with a first output shaft received withinthe first output shaft opening, the second side gear defining a secondoutput shaft opening configured to provide a second torque transmittingconnection with a second output shaft received within the second outputshaft opening; a pair of pinion gears mounted between the first andsecond side gears, both of the pinion gears being rotatably mounted on across shaft that is fixed for rotation with the differential casing; aclutch pack including a plurality of annular plates interleaved betweena plurality of annular friction disks, the clutch pack arranged on afirst side of the cross shaft; and an actuator assembly comprising apiston received in a piston housing, the actuator assembly configured toactuate the clutch pack, the actuator assembly arranged on a second sideof the cross shaft, opposite the first side.
 2. The differential gearassembly of claim 1, further comprising a plurality of transfer rodsthat couple the clutch pack and the actuator assembly.
 3. Thedifferential gear assembly of claim 2 wherein the clutch pack furthercomprises a first transfer plate and the actuator assembly furthercomprises a second transfer plate, wherein the plurality of transferrods are coupled between the first and second transfer plates.
 4. Thedifferential gear assembly of claim 1 wherein the actuator assemblyfurther comprises a retainer and a first needle roller disposed on oneside of the piston housing and a second needle roller disposed on anopposite side of the piston housing.
 5. The differential gear assemblyof claim 4 wherein the piston further comprises at least one o-ringmounted thereon, the at least one o-ring configured to slidably traversealong the piston housing.
 6. The differential gear assembly of claim 1wherein the actuator assembly further comprises a hydraulic fittingextending from an outer diameter of the piston housing, the hydraulicfitting configured to communicate hydraulic fluid from a hydraulic fluidsource to the actuator assembly.
 7. The differential gear assembly ofclaim 1 wherein the pair of pinion gears are intermeshed with the firstand second side gears to form a torque transfer arrangement configuredfor transferring torque between the pinion gears and the first andsecond side gears to rotate the first and second side gears about theaxis of rotation, the torque transfer arrangement also being configuredfor allowing the first and second side gears to rotate at differentrotational speeds with respect to one another about the axis ofrotation.
 8. The differential gear assembly of claim 1 wherein theclutch pack is generally concentric with the first side gear and theactuator assembly is generally concentric with the second side gear. 9.The differential gear assembly of claim 1 wherein the clutch pack andthe actuator assembly are configured on opposite ends of thedifferential casing.
 10. The differential gear assembly of claim 1wherein the piston includes a piston body defined around a centralpiston axis, the piston body including a central piston wall, an outerdiameter piston wall and an inner diameter piston wall and wherein thepiston housing includes a central annular housing wall, an outer annularhousing wall and an inner annular housing wall that collectively definean annular pocket that receives the piston.
 11. The differential gearassembly of claim 1 further comprising: a lug formed on one of thepiston and piston housing; and a groove defined by a groove wall on theother of the piston and the piston housing, wherein the lug is receivedby the groove and engagement of the lug and the groove wall inhibitsrotation of the piston around the central piston axis.
 12. Thedifferential gear assembly of claim 11 wherein the lug is formed on thepiston and the groove is formed on the piston housing.
 13. Thedifferential gear assembly of claim 12 wherein the lug is formed on theinner diameter piston wall and the groove is formed on the inner annularhousing wall.
 14. The differential gear assembly of claim 13 wherein thelug comprises a pair lugs configured on the inner diameter piston walland the groove comprises a pair of grooves formed on the inner annularhousing wall.
 15. The differential gear assembly of claim 14 wherein thepair of lugs are arranged in a diametrically opposed relationship on theinner diameter piston wall and wherein the pair of grooves are arrangedin a diametrically opposed relationship on the inner annular housingwall.
 16. The differential gear assembly of claim 14 wherein the outerdiameter piston wall defines a first annular groove and the innerdiameter piston wall defines a second annular groove, wherein a firsto-ring is received by the first annular groove and a second o-ring isreceived by the second annular groove, the first o-ring configured toslidably traverse along the outer annular housing wall, the secondo-ring configured to slidably traverse along the inner annular housingwall.
 17. A differential gear assembly comprising: a differential casingdefining first and second output shaft openings that are co-axiallyaligned along an axis of rotation of the differential casing; a firstand a second side gear rotatably mounted within the differential casing;a piston having a piston body defined around a central piston axis, thepiston body including a central piston wall, an outer diameter pistonwall and an inner diameter piston wall, configured to exert an axialforce on a clutch pack; a piston housing having a central annularhousing wall, an outer annular housing wall and an inner annular housingwall that collectively define an annular pocket that receives thepiston; a lug formed on one of the piston and piston housing; and agroove defined by a groove wall on the other of the piston and thepiston housing, wherein the lug is received by the groove and engagementof the lug and the groove wall inhibits rotation of the piston aroundthe central piston axis.
 18. The differential gear assembly of claim 17,further comprising the clutch pack, wherein the clutch pack includes aplurality of annular plates interleaved between a plurality of annularfriction disks, the clutch pack arranged on an opposite side of thedifferential casing as the piston.
 19. A differential gear assemblycomprising: a differential casing defining first and second output shaftopenings that are co-axially aligned along an axis of rotation of thedifferential casing; a first and a second side gear rotatably mountedwithin the differential casing; a pair of pinion gears mounted betweenthe first and second side gears, both of the pinion gears beingrotatably mounted on a cross shaft that is fixed for rotation with thedifferential casing; a clutch pack including a first transfer plate andplurality of annular plates interleaved between a plurality of annularfriction disks, the clutch pack arranged on a first side of the crossshaft; an actuator assembly comprising a second transfer plate and apiston received in a piston housing, the actuator assembly configured toactuate the clutch pack, the actuator assembly arranged on a second sideof the cross shaft, opposite the first side; and a plurality of transferrods coupled between the first and second transfer plates.
 20. Thedifferential gear assembly of claim 19 further comprising: a lug formedon one of the piston and piston housing; and a groove defined by agroove wall on the other of the piston and the piston housing, whereinthe lug is received by the groove and engagement of the lug and thegroove wall inhibits rotation of the piston around the central pistonaxis.
 21. The differential gear assembly of claim 19 wherein the firstand second side gears are co-axially aligned along the axis of rotationof the differential casing.
 22. The differential gear assembly of claim19 wherein: the first side gear defines a first shaft opening configuredto provide a first torque transmitting connection with a first outputshaft received within the first output shaft opening; and the secondside gear defines a second output shaft opening configured to provide asecond torque transmitting connection with a second output shaftreceived within the second output shaft opening.
 23. The differentialgear assembly of claim 19 wherein the actuator assembly furthercomprises a retainer and a first needle roller disposed on one side ofthe piston housing and a second needle roller disposed on an oppositeside of the piston housing.
 24. The differential gear assembly of claim19 wherein the actuator assembly further comprises a hydraulic fittingextending from an outer diameter of the piston housing, the hydraulicfitting configured to communicate hydraulic fluid from a hydraulic fluidsource to the actuator assembly.
 25. The differential gear assembly ofclaim 19 wherein the piston includes a piston body defined around acentral piston axis, the piston body including a central piston wall, anouter diameter piston wall and an inner diameter piston wall and whereinthe piston housing include a central annular housing wall, an outerannular housing wall and an inner annular housing wall that collectivelydefine an annular pocket that receives the piston.